1. Field of the Invention
Embodiments of the invention relate to semiconductor devices that provide a surge voltage protecting function.
2. Description of the Related Art
FIG. 9 is a circuit diagram of a conventional ignition device for an internal combustion engine for vehicles. This conventional ignition device for an internal combustion engine for vehicles includes an engine control unit (or an electronic control unit ECU) 21, an ignition IC (integrated circuit) 22 indicated with a square of dotted lines, an ignition coil 27, a voltage source 30, and an ignition plug 31. The ignition IC 22 employed in the ignition device for an internal combustion engine uses an insulated gate bipolar transistor (IGBT) 25 for an output stage switching element to control switching of primary side current of the ignition coil. A sensing IGBT 25a is provided having a gate and a collector that are commonly connected to the output stage IGBT 25. A current sensing resistor 26 is connected in series to the emitter of the sensing IGBT 25a. The resistor 34 in FIG. 9 indicates a wiring resistance.
Actual operation of the ignition device for an internal combustion engine will be described mainly concerning the ignition IC 22. The voltage source 30, which is connected to one terminal of the primary coil 28 of the ignition coil 27, supplies a constant voltage, for example 14 V that is a car battery voltage. The other terminal of the coil 28 is connected to the collector terminal (C terminal) of the ignition IC 22, the emitter terminal (E terminal) of the ignition IC 22 is connected to the ground, and the gate terminal (G terminal) is connected to the ECU 21. The ignition IC 22 comprises main components of the output stage IGBT 25 working as a switching element and the current control circuit 24 for the IGBT 25. The ECU 21 delivers signals for controlling ON and OFF of the output stage IGBT 25 and the sensing IGBT 25a of the ignition IC 22, and the signals are given to the G terminal of the ignition IC 22. When a voltage Vg at 5 V, for example, that is larger than the threshold value is given to the G terminal, the output stage IGBT 25 and the sensing IGBT 25a of the ignition IC 22 turn ON, while when a voltage Vg at 0 V, for example, that is smaller than the threshold value is given to the G terminal, the IGBTs 25 and 25a turn OFF.
When an ON signal Vg at 5 V is given to the G terminal from the ECU 21, the output stage IGBT 25 and the sensing IGBT 25a turn ON and decrease the voltage Vce between the C terminal and the E terminal. Thus, a collector current Ic begins to flow from the battery voltage source 30 through the primary coil 28 of the ignition coil 27 in the circuit between the C terminal and the E terminal of the ignition IC 22. The collector current Ic increases at a rate dl/dt that is determined by the inductance of the primary coil 28 and the voltage applied to the primary coil 28. After reaching a certain current value that is controlled by the control circuit 24, the collector current Ic is held at a constant current value for example 13 A. In operation of the control circuit 24 to hold the collector current Ic at a constant value, as shown in detail in FIG. 10, a voltage drop across a sensing resistor 26 is detected in proportion to the collector current Ic. The operational amplifier 36 controls the gate voltage of the MOSFET 37 such that the detected voltage drop equals a predetermined reference voltage 35. This controls the gate voltages of the output stage IGBT 25 and the sensing IGBT 25a. Thus, the collector current Ic is controlled at the constant value.
The control circuit 24 can be provided with a terminal that controls the gate voltages of the IGBT 25 and the IGBT 25a in the case of abnormal collector current and delivers warning signal to the ECU 21, though not illustrated in the FIG.
The control circuit 24 can have a built-in circuit for detecting abnormality of the coils 28 and 29 by sensing the voltage across the sensing resistor 26, though not illustrated in the FIG. The abnormality detection circuit for the coils 28 and 29 detects sharp build-up dv/dt of the collector voltage at the turn OFF time of the IGBT 25. More specifically, the gate voltage of the output stage IGBT 25 is pulled down by a predetermined voltage and the gate voltage of the IGBT is detected by the ECU 21 to detect abnormality of the coil. Alternatively, a warning signal is delivered by pulling down the voltage at the terminal that is connected to the reference voltage of the ECU 21. Japanese Unexamined Patent Application Publication No. 2009-138547 (also referred to herein as “Patent Document 4”), for example, discloses about an abnormality-detecting circuit for the coil.
Referring to FIG. 9, when an OFF signal Vg at 0 V is given from the ECU 21 to the G terminal, the output stage IGBT 25 of the ignition IC 22 is opened (or turned OFF) and the collector current Ic decreases abruptly. Corresponding to the abrupt change dl/dt of the collector current Ic, high voltage develops abruptly across the primary coil 28. At this moment, the secondary coil 29 also generates a high voltage of several tens of kilovolts, for example 30 kV, in proportion to the coil winding ratio. This voltage is applied to the ignition plug 31, which discharges at an applied voltage higher than about 10 kV.
The operation of the ignition device for an internal combustion engine will be described referring to operational waveforms in FIGS. 3A and 3B. As shown in FIG. 3A, when the gate voltage Vg reaches the threshold value and the output stage IGBT 25 turns ON, the collector current Ic begins to flow at a predetermined rate and the collector voltage Vc decreases immediately. After a certain time duration, the collector current Ic is controlled at a constant current value by the control circuit 24. The part encircled by the broken line at the right of the FIG. 3A corresponds to a transient state in the turning OFF time of the output stage IGBT 25. FIG. 3B shows this part with an expanded time scale of the abscissa.
As shown in FIG. 3B, when the gate voltage Vg falls down to a low value, 0 V, for example, below the threshold value and the output stage IGBT 25 turns OFF, the collector current Ic begins to decrease after a certain delay time. This delay time occurs due to the gate capacitance and the gate resistance 23 of the output stage IGBT 25. Accompanying the decrease of the collector current Ic, the collector voltage Vc rises abruptly. Although the increased collector voltage Vc is clamped by the Zener diode 33 indicated in FIG. 9, the discharge of the ignition plug occurs during this period.
Concerning the output stage IGBT 25, which is a switching element used in the ignition IC 22, high reliability is necessary about the withstand capability against surge voltages subjected by the IGBT 25 at the terminals of collector C, emitter E, and the gate G. The Zener diode 32 provided for the purpose of protecting the gate of the output stage IGBT 25 in FIG. 9, for example, clamps surge voltage of electrostatic discharge (ESD) from the ECU 21 generated by workers or machinery and protects the gate.
The Zener diode 32 for protecting the gate is, as shown in FIG. 4, can be a lateral Zener diode 132 formed by depositing polysilicon on the surface of the IGBT substrate in the ignition IC. This lateral polysilicon Zener diode 132 can be manufactured, for example, by the following procedure. On the n− epitaxial layer 11 indicated in FIG. 4 formed is an oxide film 5, on which a polysilicon gate electrode 6 is formed and simultaneously, a polysilicon layer for the Zener diode is deposited. Ion implantation into this polysilicon layer forms a PN junction consisting of an n+ layer 4, an n− layer 3, and a p+ layer 1. Then an anode electrode A and a cathode electrode K are formed, and the anode electrode A is connected to the emitter electrode E of the IGBT 25 and the cathode electrode K is connected to the gate electrode G at a respective common electric potential. The lateral polysilicon Zener diode 132, with a withstand voltage of 6 V, for example, and connected between the gate and the emitter, can clamp surge voltages over 6 V performing gate protection.
To meet the requirement for cost reduction, an ignition IC with a reduced chip size has been developed that includes a vertical Zener diode, not a lateral one, built-in with an IGBT. The advantages of the vertical Zener diode include a small surface area of the Zener diode owing to a PN junction of the Zener diode formed below the surface of the semiconductor surface (or the surface of the n-epitaxial layer 11). FIG. 5 is a sectional view of an essential part of an example of a compound device in a single chip comprising a Zener diode and an IGBT formed on one and the same semiconductor substrate in a self-separating structure.
In the compound device of the vertical Zener diode and the IGBT shown in FIG. 5, a p well layer 2 is formed on the surface region of the n-epitaxial layer 11 in a different place from the IGBT region. A p+ layer 1 and an n+ layer 4 are formed from the surface of this p well layer 2. On the surface of the thus formed p+ layer 1 and the n+ layer 4, an oxide film 5 and an interlayer dielectric film 7 are formed. The oxide film 5 and the interlayer dielectric film 7 on the p+ layer 1 and the n+ layer 4 are selectively processed for forming openings, and an emitter electrode (that is an anode electrode) is formed on the p+ layer 1 and a gate electrode (that is a cathode electrode) is formed on the n+ layer 4. The emitter electrode and the gate electrode perform an ohmic contact with the p+ layer 1 and the n+ layer 4, respectively. This manufacturing process forms the IGBT 25 and a vertical Zener diode 142 including the p+ layer 1, a p well layer 2, and the n+ layer 4, and having a pn junction that consists of the p well layer 2 and the n+ layer 4 and conducts electric current in the vertical direction.
Concerning a device including a Zener diode or a surge protecting element, Japanese Unexamined Patent Application Publication No. H11-284175 (also referred to herein as “Patent Document 1”) discloses a MOS type semiconductor device having a PN junction elongated for the purpose of raising surge-withstanding voltage of a lateral polysilicon Zener diode formed on a semiconductor substrate.
Japanese Unexamined Patent Application Publication No. H04-291767 (also referred to herein as “Patent Document 2”) discloses a method of eliminating the difficulties in selecting a withstand voltage of an externally provided Zener diode for voltage clamping by means of a clamping transistor for an IGBT, the clamping transistor being a bipolar transistor comprising a p+ layer, n− layer, and a p+ collector layer formed on the surface region of an n− layer.
Japanese Unexamined Patent Application Publication No. H05-129598 (also referred to herein as “Patent Document 3”) discloses a device comprising a PN junction and a depletion type MOSFET, the PN junction being formed on the same substrate of a power MOSFET in self-isolation, connected to the GND at the N side of the junction, and series-connected to the depletion type MOSFET.
In order to increase the surge withstand voltage of the lateral polysilicon Zener diode 132 shown in FIG. 4, however, operation resistance must be decreased. Nevertheless, since this lateral polysilicon Zener diode 132 has a thin film structure, small operation resistance needs an increased area of the PN junction. A large area of PN junction requires a large planar area dedicated to the lateral polysilicon Zener diode 132 on the semiconductor substrate. This causes increased chip cost.
On the other hand, in the case the vertical Zener diode 142 of FIG. 5 is used for a gate protection Zener diode 32, supposing a wiring resistance 34 between the E terminal and the ground in FIG. 9 is 0.1 ohm, the electric potential at the emitter increases by about 2 V with respect to the ground potential for a collector current of the IGBT 25 of 20 A. Due to the increase of the emitter potential, the potential of the p well 2 (indicated in FIG. 5), which is made at the same potential as that of the emitter, also increases by the same value of 2 V. In the conventional vertical Zener diode 142 shown in FIG. 5, even when the input voltage to the gate on the surface of the n+ layer 4 is 0 volts (that is the case an OFF voltage is given onto the collector (*1) of the IGBT), the emitter potential, which is at 2 V, is higher than the built-in voltage (*2) 0.7 V, for example, of the PN junction between the p well layer 2 and the n+ layer 4. This causes electric current flowing between the emitter and the gate. This current serves as a gate current for a parasitic thyristor consisting of the p substrate 9, the n+ epitaxial layer 10, the n− epitaxial layer 11, p well layer 2, and the n+ layer 4, turning ON the parasitic thyristor. As a result, uncontrollable current runs causing latch-up breakdown. When a vertical Zener diode 142 is used for the gate protection Zener diode 32 in FIG. 9, the Zener diode 32 needs to perform a bidirectional blocking characteristic to block the emitter potential raised by the current running through the wiring resistance. This bidirectional blocking characteristic needs to be achieved at a low cost. Thus, as described above, there are needs in the art for improved semiconductor devices.